Catheter-focused magnetic field induced renal nerve ablation

ABSTRACT

A flexible catheter includes a magnetically permeable element provided at its distal end. The magnetically permeable element is configured for placement within the renal artery. External coils, positionable on anterior and posterior portions of a patient in proximity to the renal artery, are coupled to a generator which energizes the external coils to create a high-frequency oscillating magnetic field in body tissue between the external coils including the renal artery and perivascular renal nerve tissue. The magnetically permeable element serves to concentrate the magnetic field in a region near the renal artery. The concentrated magnetic field induces high frequency electric current sufficient to ablate the perivascular renal nerve tissue proximate the renal artery. A cooling arrangement can be provided at the catheter&#39;s distal end and configured to provide cooling to the renal artery during ablation of the perivascular renal nerve tissue.

RELATED PATENT DOCUMENTS

This application claims the benefit of Provisional Patent Application Ser. No. 61/415,119 filed Nov. 18, 2010, to which priority is claimed pursuant to 35 U.S.C. §119(e) and which is incorporated herein by reference.

SUMMARY

Embodiments of the disclosure are directed to apparatuses and methods for ablating target tissue of the body using a concentrated magnetic field to induce high frequency electric current sufficient to ablate the target tissue. According to various embodiments, an apparatus includes a flexible elongated member having a proximal end, a distal end, and a length sufficient to access a target vessel of the body relative to a percutaneous access location. A magnetically permeable element is provided at a distal end of the elongated member. The magnetically permeable element has poor electrical conductivity and is configured for placement within the target vessel and adjacent a wall of the target vessel. One or more external coils are positionable on one or both of anterior and posterior portions of a patient in proximity to the target vessel. A generator is coupled to the external coils and configured to energize the external coils to create a high-frequency oscillating magnetic field in body tissue between the external coils including the target vessel and target tissue proximate the target vessel. The magnetically permeable element serves to concentrate the magnetic field in a region near the target vessel. The concentrated magnetic field induces high frequency electric current sufficient to ablate the target tissue proximate the vessel.

According to some embodiments, an apparatus includes a flexible elongated member having a proximal end, a distal end, and a length sufficient to access a renal artery relative to a percutaneous access location. A magnetically permeable element is provided at a distal end of the elongated member. The magnetically permeable element has poor electrical conductivity and is configured for placement within the renal artery and adjacent a wall of the renal artery. One or more external coils are positionable on one or both of anterior and posterior portions of a patient in proximity to the renal artery. A generator is coupled to the external coils and configured to energize the external coils to create a high-frequency oscillating magnetic field in body tissue between the external coils including the renal artery and perivascular renal nerve tissue proximate the renal artery. The magnetically permeable element serves to concentrate the magnetic field in a region near the renal artery. The concentrated magnetic field induces high frequency electric current sufficient to ablate the perivascular renal nerve tissue proximate the renal artery. A cooling arrangement can be provided at the distal end of the elongated member and configured to provide cooling to the renal artery during ablation of the perivascular renal nerve tissue.

In other embodiments, a method involves energizing one or more external coils positionable on one or both of anterior and posterior portions of a patient in proximity to a renal artery to create a high-frequency oscillating magnetic field in body tissue between the external coils including renal artery tissue and perivascular renal nerve tissue adjacent the renal artery. The method also involves concentrating the magnetic field in a region near the renal artery, and ablating the perivascular renal nerve tissue using high frequency electric current induced in the perivascular renal nerve tissue by the concentrated magnetic field. The method may further involve providing cooling to the renal artery during ablation of the perivascular renal nerve tissue.

These and other features can be understood in view of the following detailed discussion and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a right kidney and renal vasculature including a renal artery branching laterally from the abdominal aorta;

FIGS. 2A and 2B illustrate sympathetic innervation of the renal artery;

FIG. 3A illustrates various tissue layers of the wall of the renal artery;

FIGS. 3B and 3C illustrate a portion of a renal nerve;

FIG. 4 illustrates a medical system for ablating target tissue of the body, such as perivascular renal nerve tissue, using a concentrated magnetic field to induce high frequency electric current sufficient to ablate the target tissue in accordance with various embodiments;

FIG. 5 is a schematic representation of an externally generated magnetic field concentrated at a target region adjacent a renal artery by use of a magnetic focusing catheter in accordance with various embodiments; and

FIG. 6 illustrates an apparatus for producing ablative heating in innervated renal arterial and perivascular tissue provided from currents induced by an external magnetic field in accordance with various embodiments.

DETAILED DESCRIPTION

Ablation of perivascular renal nerves has been used as a treatment for hypertension. Radiofrequency (RF) electrodes placed in the renal artery can be used to ablate the nerves, but with risk of artery wall injury. To control injury to the artery wall, one approach is to ablate at discrete locations along and around the artery, repositioning the electrode between locations. Devices with multiple RF electrodes have been proposed to avoid the need for multiple repositioning and ablation cycles, but these devices and their use are more complicated.

Even with ablation of discrete locations, renal artery injury in these locations can occur due to local high temperatures resulting from high current density near the electrodes. Many RF ablation approaches produce a small focal area of heating, with the greatest heating at the artery wall. Away from the electrode, the current density and ohmic heating fall off very rapidly. In order to effectively ablate the target tissue, a larger zone of heating, or multiple separate ablations at different locations, is needed.

Embodiments of the disclosure are directed to apparatuses and methods for effectively ablating target tissue of the body with reduced injury to non-targeted tissue. Embodiments of the disclosure are directed to apparatuses and methods for ablation of perivascular renal nerves for treatment of hypertension. Embodiments are directed to apparatuses and methods for generating a high-frequency oscillating magnetic field in body tissue that includes target tissue to be ablated, concentrating the magnetic field in a region near a magnetically permeable element positioned proximate the target tissue, and ablating the target tissue using high frequency electric current induced in the target tissue proximate the magnetically permeable element by the concentrated magnetic field. In some embodiments, the magnetically permeable element is configured for placement within a vessel, such as the renal artery, and the target tissue is located proximate the vessel, such as perivascular renal nerve tissue adjacent the renal artery.

Various embodiments of the disclosure are directed to apparatuses and methods for renal denervation for treating hypertension. Hypertension is a chronic medical condition in which the blood pressure is elevated. Persistent hypertension is a significant risk factor associated with a variety of adverse medical conditions, including heart attacks, heart failure, arterial aneurysms, and strokes. Persistent hypertension is a leading cause of chronic renal failure. Hyperactivity of the sympathetic nervous system serving the kidneys is associated with hypertension and its progression. Deactivation of nerves in the kidneys via renal denervation can reduce blood pressure, and may be a viable treatment option for many patients with hypertension who do not respond to conventional drugs.

The kidneys are instrumental in a number of body processes, including blood filtration, regulation of fluid balance, blood pressure control, electrolyte balance, and hormone production. One primary function of the kidneys is to remove toxins, mineral salts, and water from the blood to form urine. The kidneys receive about 20-25% of cardiac output through the renal arteries that branch left and right from the abdominal aorta, entering each kidney at the concave surface of the kidneys, the renal hilum.

Blood flows into the kidneys through the renal artery and the afferent arterioles, entering the filtration portion of the kidney, the renal corpuscle. The renal corpuscle is composed of the glomerulus, a thicket of capillaries, surrounded by a fluid-filled, cup-like sac called Bowman's capsule. Solutes in the blood are filtered through the very thin capillary walls of the glomerulus due to the pressure gradient that exists between the blood in the capillaries and the fluid in the Bowman's capsule. The pressure gradient is controlled by the contraction or dilation of the arterioles. After filtration occurs, the filtered blood moves through the efferent arterioles and the peritubular capillaries, converging in the interlobular veins, and finally exiting the kidney through the renal vein.

Particles and fluid filtered from the blood move from the Bowman's capsule through a number of tubules to a collecting duct. Urine is formed in the collecting duct and then exits through the ureter and bladder. The tubules are surrounded by the peritubular capillaries (containing the filtered blood). As the filtrate moves through the tubules and toward the collecting duct, nutrients, water, and electrolytes, such as sodium and chloride, are reabsorbed into the blood.

The kidneys are innervated by the renal plexus which emanates primarily from the aorticorenal ganglion. Renal ganglia are formed by the nerves of the renal plexus as the nerves follow along the course of the renal artery and into the kidney. The renal nerves are part of the autonomic nervous system which includes sympathetic and parasympathetic components. The sympathetic nervous system is known to be the system that provides the bodies “fight or flight” response, whereas the parasympathetic nervous system provides the “rest and digest” response. Stimulation of sympathetic nerve activity triggers the sympathetic response which causes the kidneys to increase production of hormones that increase vasoconstriction and fluid retention. This process is referred to as the renin-angiotensin-aldosterone-system (RAAS) response to increased renal sympathetic nerve activity.

In response to a reduction in blood volume, the kidneys secrete renin, which stimulates the production of angiotensin. Angiotensin causes blood vessels to constrict, resulting in increased blood pressure, and also stimulates the secretion of the hormone aldosterone from the adrenal cortex. Aldosterone causes the tubules of the kidneys to increase the reabsorption of sodium and water, which increases the volume of fluid in the body and blood pressure.

Congestive heart failure (CHF) is a condition that has been linked to kidney function. CHF occurs when the heart is unable to pump blood effectively throughout the body. When blood flow drops, renal function degrades because of insufficient perfusion of the blood within the renal corpuscles. The decreased blood flow to the kidneys triggers an increase in sympathetic nervous system activity (i.e., the RAAS becomes too active) that causes the kidneys to secrete hormones that increase fluid retention and vasorestriction. Fluid retention and vasorestriction in turn increases the peripheral resistance of the circulatory system, placing an even greater load on the heart, which diminishes blood flow further. If the deterioration in cardiac and renal functioning continues, eventually the body becomes overwhelmed, and an episode of heart failure decompensation occurs, often leading to hospitalization of the patient.

FIG. 1 is an illustration of a right kidney 10 and renal vasculature including a renal artery 12 branching laterally from the abdominal aorta 20. In FIG. 1, only the right kidney 10 is shown for purposes of simplicity of explanation, but reference will be made herein to both right and left kidneys and associated renal vasculature and nervous system structures, all of which are contemplated within the context of embodiments of the disclosure. The renal artery 12 is purposefully shown to be disproportionately larger than the right kidney 10 and abdominal aorta 20 in order to facilitate discussion of various features and embodiments of the present disclosure.

The right and left kidneys are supplied with blood from the right and left renal arteries that branch from respective right and left lateral surfaces of the abdominal aorta 20. Each of the right and left renal arteries is directed across the crus of the diaphragm, so as to form nearly a right angle with the abdominal aorta 20. The right and left renal arteries extend generally from the abdominal aorta 20 to respective renal sinuses proximate the hilum 17 of the kidneys, and branch into segmental arteries and then interlobular arteries within the kidney 10. The interlobular arteries radiate outward, penetrating the renal capsule and extending through the renal columns between the renal pyramids. Typically, the kidneys receive about 20% of total cardiac output which, for normal persons, represents about 1200 mL of blood flow through the kidneys per minute.

The primary function of the kidneys is to maintain water and electrolyte balance for the body by controlling the production and concentration of urine. In producing urine, the kidneys excrete wastes such as urea and ammonium. The kidneys also control reabsorption of glucose and amino acids, and are important in the production of hormones including vitamin D, renin and erythropoietin.

An important secondary function of the kidneys is to control metabolic homeostasis of the body. Controlling hemostatic functions include regulating electrolytes, acid-base balance, and blood pressure. For example, the kidneys are responsible for regulating blood volume and pressure by adjusting volume of water lost in the urine and releasing erythropoietin and renin, for example. The kidneys also regulate plasma ion concentrations (e.g., sodium, potassium, chloride ions, and calcium ion levels) by controlling the quantities lost in the urine and the synthesis of calcitrol. Other hemostatic functions controlled by the kidneys include stabilizing blood pH by controlling loss of hydrogen and bicarbonate ions in the urine, conserving valuable nutrients by preventing their excretion, and assisting the liver with detoxification.

Also shown in FIG. 1 is the right suprarenal gland 11, commonly referred to as the right adrenal gland. The suprarenal gland 11 is a star-shaped endocrine gland that rests on top of the kidney 10. The primary function of the suprarenal glands (left and right) is to regulate the stress response of the body through the synthesis of corticosteroids and catecholamines, including cortisol and adrenaline (epinephrine), respectively. Encompassing the kidneys 10, suprarenal glands 11, renal vessels 12, and adjacent perirenal fat is the renal fascia, e.g., Gerota's fascia, (not shown), which is a fascial pouch derived from extraperitoneal connective tissue.

The autonomic nervous system of the body controls involuntary actions of the smooth muscles in blood vessels, the digestive system, heart, and glands. The autonomic nervous system is divided into the sympathetic nervous system and the parasympathetic nervous system. In general terms, the parasympathetic nervous system prepares the body for rest by lowering heart rate, lowering blood pressure, and stimulating digestion. The sympathetic nervous system effectuates the body's fight-or-flight response by increasing heart rate, increasing blood pressure, and increasing metabolism.

In the autonomic nervous system, fibers originating from the central nervous system and extending to the various ganglia are referred to as preganglionic fibers, while those extending from the ganglia to the effector organ are referred to as postganglionic fibers. Activation of the sympathetic nervous system is effected through the release of adrenaline (epinephrine) and to a lesser extent norepinephrine from the suprarenal glands 11. This release of adrenaline is triggered by the neurotransmitter acetylcholine released from preganglionic sympathetic nerves.

The kidneys and ureters (not shown) are innervated by the renal nerves 14. FIGS. 1 and 2A-2B illustrate sympathetic innervation of the renal vasculature, primarily innervation of the renal artery 12. The primary functions of sympathetic innervation of the renal vasculature include regulation of renal blood flow and pressure, stimulation of renin release, and direct stimulation of water and sodium ion reabsorption.

Most of the nerves innervating the renal vasculature are sympathetic postganglionic fibers arising from the superior mesenteric ganglion 26. The renal nerves 14 extend generally axially along the renal arteries 12, enter the kidneys 10 at the hilum 17, follow the branches of the renal arteries 12 within the kidney 10, and extend to individual nephrons. Other renal ganglia, such as the renal ganglia 24, superior mesenteric ganglion 26, the left and right aorticorenal ganglia 22, and celiac ganglia 28 also innervate the renal vasculature. The celiac ganglion 28 is joined by the greater thoracic splanchnic nerve (greater TSN). The aorticorenal ganglia 26 is joined by the lesser thoracic splanchnic nerve (lesser TSN) and innervates the greater part of the renal plexus.

Sympathetic signals to the kidney 10 are communicated via innervated renal vasculature that originates primarily at spinal segments T10-T12 and L1. Parasympathetic signals originate primarily at spinal segments S2-S4 and from the medulla oblongata of the lower brain. Sympathetic nerve traffic travels through the sympathetic trunk ganglia, where some may synapse, while others synapse at the aorticorenal ganglion 22 (via the lesser thoracic splanchnic nerve, i.e., lesser TSN) and the renal ganglion 24 (via the least thoracic splanchnic nerve, i.e., least TSN). The postsynaptic sympathetic signals then travel along nerves 14 of the renal artery 12 to the kidney 10. Presynaptic parasympathetic signals travel to sites near the kidney 10 before they synapse on or near the kidney 10.

With particular reference to FIG. 2A, the renal artery 12, as with most arteries and arterioles, is lined with smooth muscle 34 that controls the diameter of the renal artery lumen 13. Smooth muscle, in general, is an involuntary non-striated muscle found within the media layer of large and small arteries and veins, as well as various organs. The glomeruli of the kidneys, for example, contain a smooth muscle-like cell called the mesangial cell. Smooth muscle is fundamentally different from skeletal muscle and cardiac muscle in terms of structure, function, excitation-contraction coupling, and mechanism of contraction.

Smooth muscle cells can be stimulated to contract or relax by the autonomic nervous system, but can also react on stimuli from neighboring cells and in response to hormones and blood borne electrolytes and agents (e.g., vasodilators or vasoconstrictors). Specialized smooth muscle cells within the afferent arterioles of the juxtaglomerular apparatus of kidney 10, for example, produces renin which activates the angiotension II system.

The renal nerves 14 innervate the smooth muscle 34 of the renal artery wall 15 and extend lengthwise in a generally axial or longitudinal manner along the renal artery wall 15. The smooth muscle 34 surrounds the renal artery circumferentially, and extends lengthwise in a direction generally transverse to the longitudinal orientation of the renal nerves 14, as is depicted in FIG. 2B.

The smooth muscle 34 of the renal artery 12 is under involuntary control of the autonomic nervous system. An increase in sympathetic activity, for example, tends to contract the smooth muscle 34, which reduces the diameter of the renal artery lumen 13 and decreases blood perfusion. A decrease in sympathetic activity tends to cause the smooth muscle 34 to relax, resulting in vessel dilation and an increase in the renal artery lumen diameter and blood perfusion. Conversely, increased parasympathetic activity tends to relax the smooth muscle 34, while decreased parasympathetic activity tends to cause smooth muscle contraction.

FIG. 3A shows a segment of a longitudinal cross-section through a renal artery, and illustrates various tissue layers of the wall 15 of the renal artery 12. The innermost layer of the renal artery 12 is the endothelium 30, which is the innermost layer of the intima 32 and is supported by an internal elastic membrane. The endothelium 30 is a single layer of cells that contacts the blood flowing though the vessel lumen 13. Endothelium cells are typically polygonal, oval, or fusiform, and have very distinct round or oval nuclei. Cells of the endothelium 30 are involved in several vascular functions, including control of blood pressure by way of vasoconstriction and vasodilation, blood clotting, and acting as a barrier layer between contents within the lumen 13 and surrounding tissue, such as the membrane of the intima 32 separating the intima 32 from the media 34, and the adventitia 36. The membrane or maceration of the intima 32 is a fine, transparent, colorless structure which is highly elastic, and commonly has a longitudinal corrugated pattern.

Adjacent the intima 32 is the media 33, which is the middle layer of the renal artery 12. The media is made up of smooth muscle 34 and elastic tissue. The media 33 can be readily identified by its color and by the transverse arrangement of its fibers. More particularly, the media 33 consists principally of bundles of smooth muscle fibers 34 arranged in a thin plate-like manner or lamellae and disposed circularly around the arterial wall 15. The outermost layer of the renal artery wall 15 is the adventitia 36, which is made up of connective tissue. The adventitia 36 includes fibroblast cells 38 that play an important role in wound healing.

A perivascular region 37 is shown adjacent and peripheral to the adventitia 36 of the renal artery wall 15. A renal nerve 14 is shown proximate the adventitia 36 and passing through a portion of the perivascular region 37. The renal nerve 14 is shown extending substantially longitudinally along the outer wall 15 of the renal artery 12. The main trunk of the renal nerves 14 generally lies in or on the adventitia 36 of the renal artery 12, often passing through the perivascular region 37, with certain branches coursing into the media 33 to enervate the renal artery smooth muscle 34.

Embodiments of the disclosure may be implemented to provide varying degrees of denervation therapy to innervated renal vasculature. For example, embodiments of the disclosure may provide for control of the extent and relative permanency of renal nerve impulse transmission interruption achieved by denervation therapy delivered using a treatment apparatus of the disclosure. The extent and relative permanency of renal nerve injury may be tailored to achieve a desired reduction in sympathetic nerve activity (including a partial or complete block) and to achieve a desired degree of permanency (including temporary or irreversible injury).

Returning to FIGS. 3B and 3C, the portion of the renal nerve 14 shown in FIGS. 3B and 3C includes bundles 14 a of nerve fibers 14 b each comprising axons or dendrites that originate or terminate on cell bodies or neurons located in ganglia or on the spinal cord, or in the brain. Supporting tissue structures 14 c of the nerve 14 include the endoneurium (surrounding nerve axon fibers), perineurium (surrounds fiber groups to form a fascicle), and epineurium (binds fascicles into nerves), which serve to separate and support nerve fibers 14 b and bundles 14 a. In particular, the endoneurium, also referred to as the endoneurium tube or tubule, is a layer of delicate connective tissue that encloses the myelin sheath of a nerve fiber 14 b within a fasciculus.

Major components of a neuron include the soma, which is the central part of the neuron that includes the nucleus, cellular extensions called dendrites, and axons, which are cable-like projections that carry nerve signals. The axon terminal contains synapses, which are specialized structures where neurotransmitter chemicals are released in order to communicate with target tissues. The axons of many neurons of the peripheral nervous system are sheathed in myelin, which is formed by a type of glial cell known as Schwann cells. The myelinating Schwann cells are wrapped around the axon, leaving the axolemma relatively uncovered at regularly spaced nodes, called nodes of Ranvier. Myelination of axons enables an especially rapid mode of electrical impulse propagation called saltation.

In some embodiments, a treatment apparatus of the disclosure may be implemented to deliver denervation therapy that causes transient and reversible injury to renal nerve fibers 14 b. In other embodiments, a treatment apparatus of the disclosure may be implemented to deliver denervation therapy that causes more severe injury to renal nerve fibers 14 b, which may be reversible if the therapy is terminated in a timely manner. In preferred embodiments, a treatment apparatus of the disclosure may be implemented to deliver denervation therapy that causes severe and irreversible injury to renal nerve fibers 14 b, resulting in permanent cessation of renal sympathetic nerve activity. For example, a treatment apparatus may be implemented to deliver a denervation therapy that disrupts nerve fiber morphology to a degree sufficient to physically separate the endoneurium tube of the nerve fiber 14 b, which can prevent regeneration and re-innervation processes.

By way of example, and in accordance with Seddon's classification as is known in the art, a treatment apparatus of the disclosure may be implemented to deliver a denervation therapy that interrupts conduction of nerve impulses along the renal nerve fibers 14 b by imparting damage to the renal nerve fibers 14 b consistent with neruapraxia. Neurapraxia describes nerve damage in which there is no disruption of the nerve fiber 14 b or its sheath. In this case, there is an interruption in conduction of the nerve impulse down the nerve fiber, with recovery taking place within hours to months without true regeneration, as Wallerian degeneration does not occur. Wallerian degeneration refers to a process in which the part of the axon separated from the neuron's cell nucleus degenerates. This process is also known as anterograde degeneration. Neurapraxia is the mildest form of nerve injury that may be imparted to renal nerve fibers 14 b by use of a treatment apparatus according to embodiments of the disclosure.

A treatment apparatus may be implemented to interrupt conduction of nerve impulses along the renal nerve fibers 14 b by imparting damage to the renal nerve fibers consistent with axonotmesis. Axonotmesis involves loss of the relative continuity of the axon of a nerve fiber and its covering of myelin, but preservation of the connective tissue framework of the nerve fiber. In this case, the encapsulating support tissue 14 c of the nerve fiber 14 b is preserved. Because axonal continuity is lost, Wallerian degeneration occurs. Recovery from axonotmesis occurs only through regeneration of the axons, a process requiring time on the order of several weeks or months. Electrically, the nerve fiber 14 b shows rapid and complete degeneration. Regeneration and re-innervation may occur as long as the endoneural tubes are intact.

A treatment apparatus may be implemented to interrupt conduction of nerve impulses along the renal nerve fibers 14 b by imparting damage to the renal nerve fibers 14 b consistent with neurotmesis. Neurotmesis, according to Seddon's classification, is the most serious nerve injury in the scheme. In this type of injury, both the nerve fiber 14 b and the nerve sheath are disrupted. While partial recovery may occur, complete recovery is not possible. Neurotmesis involves loss of continuity of the axon and the encapsulating connective tissue 14 c, resulting in a complete loss of autonomic function, in the case of renal nerve fibers 14 b. If the nerve fiber 14 b has been completely divided, axonal regeneration causes a neuroma to form in the proximal stump.

A more stratified classification of neurotmesis nerve damage may be found by reference to the Sunderland System as is known in the art. The Sunderland System defines five degrees of nerve damage, the first two of which correspond closely with neurapraxia and axonotmesis of Seddon's classification. The latter three Sunderland System classifications describe different levels of neurotmesis nerve damage.

The first and second degrees of nerve injury in the Sunderland system are analogous to Seddon's neurapraxia and axonotmesis, respectively. Third degree nerve injury, according to the Sunderland System, involves disruption of the endoneurium, with the epineurium and perineurium remaining intact. Recovery may range from poor to complete depending on the degree of intrafascicular fibrosis. A fourth degree nerve injury involves interruption of all neural and supporting elements, with the epineurium remaining intact. The nerve is usually enlarged. Fifth degree nerve injury involves complete transection of the nerve fiber 14 b with loss of continuity.

In accordance with various embodiments, and with reference to FIG. 4, there is shown an apparatus 100 for ablating target tissue of the body. The apparatus 100 shown in FIG. 4 includes an external apparatus 101 and an apparatus 102 configured for placement within the body. The external and internal apparatuses 101 and 102 cooperate to focus an externally generated magnetic field at a target region within the body and intensify the magnetic field near the target region resulting in increased induced electric current and ohmic heating in target tissue within or near the target region. The ohmic heating is preferably controlled so that the target tissue is thermally ablated, but non-targeted tissue is only negligibly affected.

As discussed previously above, conventional RF ablation approaches produce a small focal area of heating, with the greatest heating at the artery. The current density and ohmic heating fall off by the 4^(th) power of the distance from the electrode center. Embodiments of the disclosure are directed to approaches that generate a more uniform heating in the region adjacent to the artery, allowing more complete ablation of target tissue with less artery wall injury.

According to the embodiment shown in FIG. 4, the internal apparatus 102 includes a flexible elongated member 104 having a proximal end, a distal end, and a length sufficient to access a target vessel of the body, such as a renal artery 12, relative to a percutaneous access location. A magnetically permeable element 120 is provided at a distal end of the elongated member 104. The magnetically permeable element 120 preferably has low or poor electrical conductivity and is configured for placement within the renal artery 12 and adjacent a wall of the artery 12. The internal apparatus 102 shown in FIG. 4 is preferably delivered to the renal artery 12 via the inferior aorta 20 using one or both of a guiding catheter and a delivery sheath. Alternatively, the internal apparatus 102 can be implemented as a guiding catheter, and may incorporate a lumen through which a guidewire can pass for advancing the apparatus 102 through vasculature via an over-the-wire delivery technique, for example. Marker bands or features can be placed on one or multiple parts of the elongated member 104, the magnetically permeable element 120, and/or a support arrangement 122 that contains or supports the magnetically permeable element 120. The marker bands or features aid the clinician in determining the location and position of specific portions of the elongated member 104, such as the magnetically permeable element 120.

The external apparatus 101 includes one or more external coils 105A and 105B that can be positioned on one or both of anterior and posterior portions of a patient in proximity to the renal artery 12. The external apparatus 101 further includes a generator (shown in FIG. 6) coupled to the external coils 105A and 105B. The generator is configured to energize the external coils 105A and 105B to create a high-frequency oscillating magnetic field in body tissue between the external coils 105A and 105B including the renal artery 12 and perivascular renal nerve tissue proximate the renal artery 12. The magnetically permeable element 120 serves to concentrate the magnetic field in a region near the renal artery 12. The concentrated magnetic field induces high frequency electric current sufficient to ablate the perivascular renal nerve tissue proximate the renal artery 12.

The internal apparatus 102 may include a support arrangement 122 at the distal end of the elongated member 104. The support member 122 preferably provides support for, or containment of, magnetic material of the magnetically permeable element 120. The magnetically permeable element 120 may, for example, incorporate a vessel 122 filled with fluid comprising magnetic material. The magnetically permeable element 120 may incorporate a balloon 122 configured to receive a fluid comprising magnetic material from a lumen arrangement of the elongated member 104. The magnetically permeable element 120 can incorporate a balloon 122 formed from a composite polymeric material comprising magnetic material or a polymeric balloon 122 comprising a coating of nonconductive or poorly conductive magnetic material. The magnetically permeable element 120 may incorporate a balloon 122, and at least a circumferential region of the balloon 122 is heated by the induced electric current, such that a complete circumferential region of the perivascular renal nerve tissue is ablated. The magnetically permeable element 120 may include a solid element or a coated element 120 comprising nonconductive or poorly conductive magnetic material. The magnetically permeable element 120 can incorporate a tube 122 containing magnetic material. Other configurations of the magnetically permeable element 120 and support arrangement 122 are contemplated.

Referring now to FIG. 5, there is illustrated a schematic representation of an externally generated magnetic field 110 which is being concentrated at a target region of the body adjacent a renal artery 12 by use of a magnetic focusing catheter 102 according to various embodiments of the disclosure. FIG. 5 shows a cross-section of the renal artery 12 within which a spherical vessel 122 containing a ferrofluid 120 is positioned. In FIG. 5, the renal artery 12 is oriented coming out of the page. The electric field lines 135 around the spherical vessel 122 containing the ferrofluid 120 are illustrated for simplicity. The spherical vessel 122 that contains a ferrofluid 120 represents one of many possible configurations of a magnetically permeable element 120 and support arrangement 122, and is shown for non-limiting illustrative purposes. For example, a cylindrical or annular ferrofluid volume may be provided at a distal end or tip of a catheter shaft 104, with the current loops displaced to the side since the ferrofluid is nonconductive. In FIG. 5, the magnetic field 110 is illustrated as being concentrated by a factor of about 2 for clarity of illustration. However, concentration of the magnetic field 115 proximate the ferrofluid filled vessel 122 by ratios of magnetic permeability in the range of about 5 to 15 are typically achieved.

With reference to FIGS. 4 and 5, and in accordance with various embodiments, a balloon catheter 102 is placed in a renal artery 12 and the balloon 122 is filled with a ferrofluid 120. External coils 105A and 105B are placed on the front and the back of the patient and energized to create a high-frequency oscillating magnetic field 110 in the tissue between the coils 105A and 105B. The ferrofluid 120 concentrates the magnetic field 115 in the region near the catheter balloon 122 preferably by approximately a factor of 10. The changing magnetic field 115 induces a corresponding electric field 135 and results in high frequency electric current in the tissues adjacent the balloon 122. The current causes local resistive heating in the tissue, proportional to the square of the current, resulting in local heating near the balloon 122 approximately 100 times that resulting in the other areas of tissue subject to the magnetic field 110.

In this representative embodiment, tissue around the entire circumference of the balloon 122 is heated by the induced current, avoiding the need for repeated catheter positioning and activation as with typical RF devices. The magnetically induced heating is also more uniform, since the current is affected primarily by the local magnetic field strength, which can fall off relatively slowly with separation from a magnetically permeable balloon 122. More uniform heating in the region adjacent to the renal artery 12 is achieved, allowing more complete ablation of perivascular renal nerve tissue with less artery wall injury.

The following comparison between conventional RF catheter approaches and ablation approaches consistent with embodiments of the disclosure is provided below for illustrative purposes. Conventional RF catheter approaches involve pressing a tip electrode against the arterial tissue and application of a given dose of RF power. Ablation current flows into the arterial tissue from a portion of the tip electrode. The impedance between the tip electrode and ground pad consists of contact resistance, tissue resistance, and tissue capacitance. To estimate some ablation parameters, consider a simplified model of the RF ablation. The ablation tip can be modeled as a spherical electrode surrounded by tissue having uniform electrical resistivity. It is assumed for illustrative purposes that about ⅓ of the current passes into the arterial wall, while the remaining current is lost to the blood. The voltage at a point in arterial tissue a distance “r” from the spherical tip electrode is given by:

V=V ₀(r ₀ /r)  (1)

where

-   -   V=voltage at distance r in volts     -   V₀=voltage applied to tip electrode, volts     -   r₀=tip electrode radius, meters     -   r=distance from center of tip electrode to point in tissue, in         meters         The electric field developed in the tissue is given by:

E=dV/dr=−V ₀(r ₀ /r ²)  (2)

where the electric field is directed radial outward from the tip electrode. The current density is then given by Ohms law:

J=E/ρ=V ₀(r ₀ /ρr ²)  (3)

where J=current density in Amps/m²

-   -   ρ=resistivity in Ohm-m         The total power dissipated in the arterial wall is given by:

P _(w) =∫d ³ rJE  (4)

where P_(w)=power dissipated in artery wall in Watts

-   -   ∫d³r=integral over the tissue volume

To estimate the power dissipated in the arterial wall, an estimate is made that about ⅓ of the tip electrode area is against the artery wall, so the result is about one third of the integration over all of space. Since the integrand falls off with the inverse fourth power of distance, most of the heat is dissipated close to the electrode, and the integration can be extended over all space with the result approximately equal to the heat dissipated in tissue near the tip electrode. Substituting Equation (2) and (3) into Equation (4) above, and carrying out the integration yields:

P _(w)=(4πr ₀/6ρ)V ₀ ²  (5)

where a factor of three reduction is included to account for the arterial wall contacting one third of the tip electrode area, and a factor of two reduction is included to account for RMS value of the sine wave, RF current.

The resistivity of muscle and blood are similar, about 2 Ohm-m. The tip electrode for renal nerve ablation has a diameter of about 1.5 mm. The nominal voltage amplitude during an RF ablation is about 40 volts (varies with tissue impedance for constant power ablations). Using these values in Equation (5) above yields P_(w)=1.3 Watts. This means that 2.6 Watts is dissipated in blood. The constant power setting in the RF ablation is 8 Watts. Thus, about half of this power, or 4.1 Watts, is dissipated in the leads and contact resistance between the tip electrode and tissue.

In accordance with embodiments of the disclosure, ablative heating of innervated renal arterial and perivascular tissue is provided from currents induced by an external magnetic field 110. In accordance with the representative geometry schematically illustrated in FIG. 5, a voltage is induced in tissue adjacent the magnetically permeable ferrofluid given by:

V=dΦ/dt=ωμB ₀ A  (6)

where

-   -   V induced voltage in volts     -   Φ=magnetic flux in webers     -   ω=2πf, where f is the frequency of the sine wave applied         magnetic field in Hz     -   μ=magnetic permeability of the ferrofluid     -   B₀=amplitude of the applied field in Tesla     -   A=area of a surface near the balloon

To simplify the calculation, consider currents induced to flow around the circumference of a circle of diameter “d” that lies in the plane perpendicular to the applied magnetic field 110 of FIG. 5, with the plane of the circle near the ferrofluid 120. The voltage induced around this circle is given by Equation (7) below as follows:

V=(π²/2)μfB ₀ d ²  (7)

The resistance around the circular path of tissues lying within a circle of diameter “d” is estimated by a circular current path with length equal to the circumference at half of the circle radius, or πd/2, and an area equal to the radius of the circle times the width, w, of the conductive tissue, or

R=πρ/w  (8)

The heat generated in the tissue due to the induced currents is then given by:

P=V ²/2R=[(π²/2)μfB ₀ d ²]² w/2πρ  (9)

Using the estimate from the RF ablation analysis below Equation (5) that the Ohmic power needed to ablate the tissue is 1.3 Watts, and taking the diameter of the circle equal to the width of the tissue ablated and both equal to 4 mm (as estimates of the tissue volume ablated in renal nerve RF ablation, taken from Haines, Biophysics of Radiofrequency Lesion Formation, Ch. 1, FIGS. 1-2), then solving Equation (9) above for μ f B₀ gives:

μfB ₀=8×10⁵  (10)

The permeability generally decreases with increasing frequency. As an estimate of this product, it is assumed that μ=10 at f=500 kHz can be achieved. In this case, the amplitude of the applied magnetic field needed for ablation by the induced currents is on the order of 0.15 Tesla. This is a large, but achievable magnetic field using water cooled coils 105A and 105B, for example. Coils 105A and 105B may be placed both above and below the patient over the approximate location of the renal artery 12, as shown in FIG. 4.

Tissue that is in the magnetic field but not adjacent the ferrofluid 120 will have induced currents with μ=1 in Equation (9) above. In this illustrative example, this will result in heating at a power level that is 100 times less than the tissue adjacent the ferrofluid 120, and presumably well below the level needed to significantly raise the temperature of these tissues. Also, the mixture of conductive and non-conductive body tissues, for example fat, may greatly reduce the conductivity of adjacent tissues, and may break up or prevent closed current paths in other tissues.

The power dissipated in the tissue adjacent the ferrofluid filled balloon 122 can be estimated by Equation (9) above. The heating power increases with the square of frequency, so higher frequency is desired, provided that the permeability is adequate. Ferrofluids 120 that consist of particles having a single magnetic domain may have higher permeability at high frequencies than multi-domain particle.

The magnetic particles in the ferrofluid 120 should be physically small to avoid induction of electrical currents within the particles that would generate heat within the balloon 122. Equation (9) above shows that the heating power is proportional to the fourth power of particle diameter, so conventional ferrofluids 120 having sub-micron sized particles should not heat, even though they are much less electrically resistive than body tissues. The particles need to be coated with an electrical insulating material, and suspended in an electrically insulating solvent.

A ferrofluid 120 having a large permeability compared to body tissues is desired so that heating occurs around the ferrofluid balloon 122, but not in other body tissues. Additionally, induced electric fields 135 that are spatially uniform in non-magnetic body tissues can be created, but non-uniform near the ferrofluid 120. This may be achieved, for example, using the rotating magnetic field of a bird cage winding (e.g., MRI RF bird cage), which creates a spatially uniform rotating electric field.

Near the ferrofluid balloon 122 the magnetic field 115 is distorted, and time varying electric fields 135 appear that circulate in the tissue near the balloon 122 at the rotation rate of the applied field. Heating will occur preferentially in tissues around the balloon 122 but not in tissues away from the balloon 122. Various distortions 115 of the applied field 110 and various patterns of induced electric currents in body tissue are contemplated using both paramagnetic materials and diamagnetic materials (e.g., bismuth) in geometries that optimize heating. Solid magnetic materials may be coated onto a balloon or delivered within a tube that can be coiled up against the artery wall.

In accordance with other embodiments, a balloon 122 can be inflated with ferrofluid 120. A double balloon 122 can be used in some embodiments, with the annulus inflated with ferrofluid 120. The inner balloon can be inflated with a conventional fluid. In other embodiments, a thin non-conductive coating of magnetic material can be used. The balloon 122 or other structure (e.g., catheter shaft 104 or expandable element supported at the distal end of the catheter shaft 104) can be covered with the thin non-conductive coating of magnetic material.

In various embodiments, a balloon 122 can be constructed of magnetic nanoparticles in a polymer matrix to produce a nonconductive magnetic balloon. The balloon 122 may be cooled to prevent heating of the arterial wall adjacent the balloon 122. Cooling may be provided by blood flowing through the center of the balloon (e.g., a perfusion balloon). Other cooling mechanisms include circulating cooling fluids and application of cryogenic fluids as are known in the art. The balloon 122 may contain a temperature sensor or thermometer 127 (shown in FIG. 6), for example one or more thermocouples, to monitor the temperature adjacent the balloon 122 and provide feedback to automatically or semi-automatically control the strength of the external magnetic field 110 during ablation.

According to some embodiments, and as an alternative to a ferrofluid balloon 122, a catheter 102 tipped with a ferrite or other magnetically permeable material having low electrical conductivity may be used. The ferrite is preferably pressed against the wall of the renal artery 12, and the external magnetic field 110 is applied. A thermometer 127 can be placed adjacent the artery wall in a hole through the ferrite tip to titrate the external field. A generally uniform magnetic field 110 can be obtained from external coils 105A and 105B, or a more concentrated magnetic field can be obtained with modified external components. The magnetic material on the catheter 102 can be used to orient the external components to maximize the field strength at the target region.

Turning now to FIG. 6, there is illustrated an apparatus 100 for producing ablative heating in innervated renal arterial and perivascular tissue provided from currents induced by an external magnetic field in accordance with various embodiments. In FIG. 6, the apparatus 100 includes a generator 125 electrically coupled to a pair of external coils 105A and 105B positioned on respective anterior and posterior portions of a patient in proximity to the renal artery 12. The external coils 105A and 105B may incorporate fluid channels or be thermally coupled to fluid channels for cooling the external coils 105A and 105B. The fluid channels of the external coils 105A and 105B are shown fluidly coupled to a coolant source 160 via lines 162A and 162B, respectively. Each of the coolant lines 162A and 162B preferably includes a supply line and a return line, with each of the supply lines fluidly coupled to a pump 162 of the coolant source 160.

FIG. 6 further shows a magnetic focusing catheter 102 which includes a magnetically permeable element 120. In some embodiments, the magnetically permeable element 120 is supported by, or contained within, a support arrangement 122 (e.g., balloon, vessel, tube). In embodiments that employ a balloon 122, an inflation lumen 106 is provided which extends along the length of the shaft 104 and is fluidly coupled to a pump 162 via a line 113. The support arrangement 122 is configured to position the magnetically permeable element 120 against the wall of the renal artery 12. The support arrangement 122 or other portion of the distal end of the shaft 104 preferably incorporates a perfusion or diversion apparatus to allow blood to flow through the renal artery 12 during the ablation procedure. In some embodiments, as previously discussed, the perfusion or diversion apparatus can be used to provide cooling to the renal artery wall during ablation.

In other embodiments, the support arrangement 122 or other structure at the distal end of the shaft 104 can be configured to receive a coolant or cryogen from an external coolant source 160 via a line 112 and a coolant lumen 108 of the shaft 104. For example, the support arrangement 122 can incorporate a cyroballoon or cryotube. A cryogen can be delivered from the coolant source 160 to the cyroballoon or cryotube of the support arrangement 122. In some implementations, a cryoballoon or cryotube can be configured to receive a liquid biocompatible cryogen, such as cold sterile saline or cold Ringer's solution, which is expelled into the blood flowing through the renal artery 12. In other implementations, the cryoballoon or cryotube can be constructed to provide phase-change cryothermal cooling by incorporating one or more orifices or narrowings to induce a phase change in a liquid cryogen supplied to the cryoballoon (e.g., Joule-Thomson cooling). Spent gas resulting from the phase change of the liquid cryogen is exhausted through an outlet fluidly coupled to an exhaust lumen extends to the proximal end of the catheter shaft 104. One or more temperature sensors 127 may be provided at the support arrangement 122 for measuring temperature approximating that of the renal artery wall adjacent the magnetically permeable element 120.

The generator 125 includes an oscillator configured to energize the external coils 105A and 105B to create a high-frequency oscillating magnetic field in body tissue between the external coils 105A and 105B including the renal artery 12 and perivascular renal nerve tissue proximate the renal artery 12. The generator 125 may include or be coupled to a controller 129 which controls the generator 125 in an automatic mode, semi-automatic mode or manual mode. The controller 129 is preferably coupled to one or more temperature sensors 127 which generate sensor signals indicative of the temperature at the renal artery wall adjacent the magnetically permeable element 120. The controller 129 can be programmed to automatically adjust the magnetic field produced between the external coils 105A and 105B based on the sensor signals received from the temperature sensors 127.

In general, when renal artery nerve tissue temperatures rise above about 113° F. (50° C.), protein is permanently damaged (including those of renal nerve fibers). If heated over about 65° C., collagen denatures and tissue shrinks. If heated over about 65° C. and up to 100° C., cell walls break and oil separates from water. Above about 100° C., tissue desiccates. According to some embodiments, the generator 125 is configured to control the magnetic field produced between the external coils 105A and 105B in response to signals received from temperature sensors 127 so that an intensified magnetic field created near the magnetically permeable element 120 causes heating of the perivascular renal nerve tissue 14 proximate the magnetically permeable element 120 to at least a temperature of 55° C.

Various embodiments disclosed herein are generally described in the context of ablation of perivascular renal nerves for control of hypertension. It is understood, however, that embodiments of the disclosure have applicability in other contexts, such as performing ablation from within other vessels of the body, including other arteries, veins, and vasculature (e.g., cardiac and urinary vasculature and vessels), and other tissues of the body, including various organs. Embodiments of the disclosure can be used for ablation of perivascular renal nerves, or for ablation of other tissues where a ferrofluid-filled catheter or elongated member or trocar can be placed for focusing the external magnetic field (e.g., diseased tissue, tumors, and organs). For example, for cardiac arrhythmia ablation, a catheter can be introduced through the vasculature and positioned in the left atrium. In another example, for treatment of BPH (benign prostatic hyperplasia), a catheter can be introduced through the urethra and positioned in the prostate.

It is to be understood that even though numerous characteristics of various embodiments have been set forth in the foregoing description, together with details of the structure and function of various embodiments, this detailed description is illustrative only, and changes may be made in detail, especially in matters of structure and arrangements of parts illustrated by the various embodiments to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

1. An apparatus, comprising: a flexible elongated member comprising a proximal end, a distal end, and a length sufficient to access a target vessel of the body relative to a percutaneous access location; a magnetically permeable element provided at a distal end of the elongated member, the magnetically permeable element having poor electrical conductivity and configured for placement within the target vessel and adjacent a wall of the target vessel; one or more external coils positionable on one or both of anterior and posterior portions of a patient in proximity to the target vessel; and a generator coupled to the external coils and configured to energize the external coils to create a high-frequency oscillating magnetic field in body tissue between the external coils including the target vessel and target tissue proximate the target vessel; the magnetically permeable element serving to concentrate the magnetic field in a region near the target vessel, the concentrated magnetic field inducing high frequency electric current sufficient to ablate the target tissue proximate the vessel.
 2. The apparatus of claim 1, wherein the magnetically permeable element comprises a vessel filled with fluid comprising magnetic material.
 3. The apparatus of claim 1, wherein the magnetically permeable element comprises a balloon, the balloon configured to receive a fluid comprising magnetic material.
 4. The apparatus of claim 1, wherein the magnetically permeable element comprises a balloon, the balloon formed from a composite material comprising magnetic material or comprising a coating of nonconductive or poorly conductive magnetic material.
 5. The apparatus of claim 1, wherein: the magnetically permeable element comprises a balloon; at least a circumferential region of the balloon is heated by the induced electric current; and a complete circumferential region of the target tissue in proximity to the circumferential region of the balloon is ablated.
 6. The apparatus of claim 1, wherein the magnetically permeable element comprises a solid element or a coated element, the solid or coated element comprising nonconductive or poorly conductive magnetic material.
 7. The apparatus of claim 1, wherein the magnetically permeable element comprises a tube containing magnetic material.
 8. The apparatus of claim 1, wherein the magnetically permeable element comprises paramagnetic material and diamagnetic material arranged in geometries that enhance heating of the target tissue.
 9. The apparatus of claim 1, wherein the magnetically permeable element is configured to cooperate with an external bird cage coil that generates a rotating magnetic field, the magnetically permeable element configured to distort a magnetic field proximate the element so that time varying electric fields are produced that circulate in the target tissue at the rotation rate of rotating magnetic field.
 10. The apparatus of claim 1, further comprising a temperature sensor arranged to sense temperature proximate the magnetically permeable element.
 11. The apparatus of claim 1, further comprising a cooling arrangement at the distal end of the elongated member and configured to provide cooling to the target vessel during ablation of the target tissue.
 12. The apparatus of claim 1, wherein the magnetically permeable element is configured to create a circumferential lesion in the target tissue.
 13. The apparatus of claim 1, wherein the concentrated magnetic field is approximately 5-15 times greater than that in other regions of the body tissue between the external coils.
 14. The apparatus of claim 1, wherein the induced current causes local resistive heating near the magnetically permeable element approximately ≧100 times that in other areas of the body tissue subject to the magnetic field.
 15. The apparatus of claim 1, further comprising a cooling arrangement configured to cool the one or more external coils.
 16. An apparatus, comprising: a flexible elongated member comprising a proximal end, a distal end, and a length sufficient to access a renal artery relative to a percutaneous access location; a magnetically permeable element provided at a distal end of the elongated member, the magnetically permeable element having poor electrical conductivity and configured for placement within the renal artery and adjacent a wall of the renal artery; one or more external coils positionable on one or both of anterior and posterior portions of a patient in proximity to the renal artery; and a generator coupled to the external coils and configured to energize the external coils to create a high-frequency oscillating magnetic field in body tissue between the external coils including the renal artery and perivascular renal nerve tissue proximate the renal artery; the magnetically permeable element serving to concentrate the magnetic field in a region near the renal artery, the concentrated magnetic field inducing high frequency electric current sufficient to ablate the perivascular renal nerve tissue proximate the renal artery.
 17. The apparatus of claim 16, further comprising a cooling arrangement at the distal end of the elongated member and configured to provide cooling to the renal artery during ablation of the perivascular renal nerve tissue.
 18. The apparatus of claim 16, further comprising a cooling arrangement configured to cool the one or more external coils.
 19. The apparatus of claim 16, wherein the magnetically permeable element is configured to create a circumferential lesion in the perivascular renal nerve tissue.
 20. The apparatus of claim 16, wherein: the magnetically permeable element comprises a balloon; and the balloon comprises a cooling arrangement configured to provide cooling to the renal artery during ablation of the perivascular renal nerve tissue.
 21. A method, comprising: energizing one or more external coils positionable on one or both of anterior and posterior portions of a patient in proximity to a renal artery to create a high-frequency oscillating magnetic field in body tissue between the external coils including renal artery tissue and perivascular renal nerve tissue adjacent the renal artery; concentrating the magnetic field in a region near the renal artery; and ablating the perivascular renal nerve tissue using high frequency electric current induced in the perivascular renal nerve tissue by the concentrated magnetic field.
 22. The method of claim 21, wherein: concentrating the magnetic field comprises concentrating the magnetic field in a region near a magnetically permeable element situated in the renal artery; and ablating the perivascular renal nerve tissue comprises ablating the perivascular renal nerve tissue using high frequency electric current induced in the perivascular renal nerve tissue proximate the magnetically permeable element by the concentrated magnetic field.
 23. The method of claim 21, further comprising providing cooling to the renal artery during ablation of the perivascular renal nerve tissue. 